Conventional diagnostic use of x-radiation includes the form of radiography, in which a still shadow image of the patient is produced on x-ray film, fluoroscopy, in which a visible real time shadow light image is produced by low intensity x-rays impinging on a fluorescent screen after passing through the patient, and computed tomography (CT) in which complete patient images are electrically reconstructed from x-rays produced by a high powered x-ray tube rotated about a patient's body.
In a typical x-ray tube, electrons are generated from a filament coil heated to thermionic emission. The electrons are accelerated as a beam from a cathode through an evacuated chamber defined by a glass envelope, toward an anode. When the electrons strike the anode with large kinetic energies and experience a sudden deceleration, x-radiation is produced. An x-ray tube assembly is contained in a housing which includes a window transmissive to x-rays, such that radiation from the anode passes through the window toward a subject undergoing examination or treatment.
Most x-ray tube designs employ filaments as a source of electrons. A filament is a coil of wire which is electrically energized so that electrons are thermionically emitted from the filament. The electrons are accelerated toward the anode due to a DC electrical potential difference between the cathode and the anode. Often this electrical potential difference is of the order of 150,000 volts, (.+-.75,000 volts to ground) necessitating significant electrical insulation between the various tube components.
In some low power x-ray tubes, electrons from a cathode filament are drawn at a high voltage to a stationary target anode. The impact of the electrons causes the generation of x-rays as well as significant thermal energy. In higher power x-ray tubes the thermal energy produced at the stationary target anode often becomes so large that the generated heat became a limiting factor in x-ray tube performance.
In order to distribute the thermal loading and reduce anode temperature a rotating anode configuration has been adopted for many applications. In this configuration, an electron beam is focused near a peripheral edge of the anode disk at a focal spot. As the anode rotates, a different portion of a circular path around the peripheral edge of the anode passes through the focal spot where x-rays are generated. Each portion along the circular path is heated to a very high temperature during the generation of x-rays and cooled as it is rotated before returning for the generation of x-rays. As higher power x-ray tubes are developed, the diameter and the mass of the rotating anode continues to grow. Further, when x-ray tubes are combined with conventional CT scanners, a gantry holding the x-ray tube is rotated around a patient's body in order to obtain complete images of the patient. Today, typical CT scanners revolve the x-ray tube around the patient's body at a rate of between 60-120 rotations-per-minute (RPM). In order for the x-ray tube to properly operate, the anode needs to be properly supported and stabilized from the effects of its own rotation and, in some instances, from centrifugal forces created by rotation of the x-ray tube about a patient's body.
Typically, the anode is mounted on a stem and rotated by a motor. The anode, stem and other components rotated by the motor are part of a rotating assembly which is supported by a bearing assembly. The bearing assemblies found in most x-ray tubes today utilize either a cantilevered bearing arrangement or a straddle bearing arrangement. In a cantilevered bearing arrangement, all bearings are located on the same side of the rotating assembly's center of mass. In a straddle bearing arrangement, bearings are located on both sides of the rotating assembly's center of mass.
One drawback to using the cantilevered bearing arrangement is that a bearing closest to the anode experiences a much greater load than the bearing(s) further from the anode. The bearing closest to the anode therefore has greater contact stresses which deleteriously effects the life of the entire bearing assembly and thus the x-ray tube life. If the size of the bearings closest to the anode were increased to distribute the contact stresses, the internal surface speeds of this bearing would increase and the bearing life would decrease due to a faster wear rate. Thus, the bearing closest to the anode would still typically fail first.
In an effort to more equally distribute the load of the rotating assembly among the bearings, the straddle bearing arrangement was developed. Typical straddle bearing arrangements employ a large bearing-to-bearing distance. The bearing-to-bearing distance is sometimes referred to as a straddle or wheelbase. The large wheelbase is required to thermally insulate the bearings from the anode which is typically very hot. The anode is often in the range of 1200 degrees C. Heat from the anode is thermally conducted to the bearings through the predominantly metal bearing assembly.
In conventional straddle bearing designs, heat transferred from the anode substantially equally effects each bearing on either side of the anode. This is the case since the bearings are typically symmetrically spaced an equidistance from the anode's center of mass in order to share the load equally, and since the thermally conductive path between the anode and each bearing is the same length. Because each bearing on either side of the anode must be moved out an equal distance from the anode's center of mass for thermal insulation purposes, the wheelbase of a conventional straddle bearing assembly is typically much larger than a wheelbase found in a cantilevered bearing arrangement. As discussed above, bearings in a cantilevered bearing arrangement are all on the same side of the anode. Thus, in a cantilevered bearing arrangement, once the bearing closest to the anode is thermally insulated the other bearing(s) can be placed at an appropriate distance further away from the bearing closest to the anode. This is possible since the thermally conductive path to the other bearings is always further than the thermally conductive path to the bearing closest to the anode. Therefore, thermal insulation does not require the large wheelbase in a cantilevered bearing arrangement that it does in a conventional straddle bearing arrangement.
An unfortunate drawback to having a large wheelbase is that thermal compensation becomes much more difficult. Thermal compensation relates to the accommodations made in the bearing assembly in both the radial and axial directions to account for changes in bearing tolerances caused by temperature variances. The larger the wheelbase, the more thermal growth and shrinkage the bearing assembly design must be able to withstand. Thus, designing for thermal compensation in a straddle bearing assembly is extremely difficult given the large wheelbases dictated by the need to thermally insulate the bearings.
One common technique used in both cantilevered and straddle bearing arrangements to ensure predictability in the effect temperature swings have on the bearing assembly is to only allow thermal movement in the bearing assembly to occur in one direction as opposed to compensating for thermal movement symmetrically about the bearing. This is typically done by securing in place at least one end of each component of the bearing assembly such that thermal shrinkage and growth occurs in a known direction at the opposite end. As a consequence, as components coupled to the bearing assembly expand and contract due to temperature variances, the anode also moves thereby creating changes to the focal spot. More specifically, as most conventional bearing assemblies restrict thermal expansion and contraction to occur in a direction substantially parallel with an axis of rotation of the anode, thermal movements typically cause the focal spot to change is size. Such change in size to the focal spot is undesirable as it causes blurring to images taken from the x-rays radiating from the anode. Further, such thermal expansion and contraction also causes undesired movement of the focal spot with respect to x-ray detectors outside of the x-ray tube which may additionally deleteriously effect the quality of the images taken.
Typical implementations of straddle bearing designs also employ an outer bearing race rotation. Inner bearing race rotation is not available in straddle bearing designs as aligning bearings on opposite sides of the anode to handle such inner bearing race rotation has not been achievable. Aligning the bearings is difficult primarily because outer races for each bearing must be independently positioned on opposite sides of the anode in conventional straddle bearing designs and slight deviations from perfectly symmetrically placement of the outer bearings causes the anode supported by the bearing assembly to wobble during operation. Unfortunately, outer bearing race rotation increases surface speeds in the bearing and therefore increases the wear on the bearings. Further, because bearings in a straddle bearing assembly are physically located on both sides of the anode, difficulties arise in electrically isolating the bearings from high voltages. Specifically, if an x-ray tube is configured in a bi-polar arrangement, the cathode would be at a -75,000 volt potential while the anode would be at a +75,000 volt potential. As the bearing assembly is coupled to the anode assembly, the bearings are at the anode voltage potential. However, in a conventional straddle bearing assembly, at least one of the bearings is in close proximity to the cathode and therefor needs to be electrically insulated from the cathode voltage potentials in order to avoid undesirous arcing from occurring. As insulating the bearing from the cathode voltage potential is normally too difficult to accomplish, x-ray tube having a straddle bearing assembly typically implement a single ended configuration where the anode is at ground potential and the cathode is at -150,000 volts. Unfortunately this makes it difficult for such x-ray tubes to be used in a retrofit manner since most x-ray tube generators are configured to handle only a bi-polar topology.
Therefore, what is needed is a bearing assembly wherein each bearing of the bearing assembly is capable of supporting a substantially equal load while still overcoming the shortfalls discussed above related to both cantilevered and straddled bearing assemblies.